Melittin is a polycationic 26 aminoacid peptide isolated from bee venom. It is a powerful cytolysin whose effects are mediated by perturbation of the lipid-bilayer component of the plasma membranes of cells. Melittin is capable of forming an amphipathic helix upon interaction with hydrophobic surfaces and it is believed that this property determines its ability to penetrate into phospholipid membranes. Our studies to date have shown that the peptide forms apparently unique complexes with saturated phospholipids such as dimyristoylglycerophosphorylcholine and dipentadecanoylglycerophosphorylcholine that are characterized by being non-vesicular, and whose size and thermotropic properties are dependent on the melittin-lipid ratios. The thermotropic behavior suggests that the lipid remains in the form of a bilayer in the lipid:peptide "particle". The kinetics of interaction are apparently complex with respect to changes in conformation of the peptide and alterations in lipid order and dynamics. We intend now to apply a variety of biophysical techniques to fully characterize the form, structure and dynamcis of these and other melittin-lipid complexes (with respect to both peptide and lipid) and to elucidate the kinetics of complex formation per se, and of lysis of vesicles. Mainly fluorescence spectroscopy, quasi-elastic light scattering, and differential scanning calorimetry will be employed for investigation of structure and dynamics. Kinetic measurements will be made in a conventional fluorometer (for fluorescence or 90 degrees light scattering measurements) or in a stopped-flow rapid scan spectrofluorometer (for rapid fluorescence or light scattering changes), but other methods will be used as indicated to answer specific questions. We hypothesize that the MLT-lipid interactions exemplify how other cytolysins elaborated by pathogens (e.g., streptolysin S, from streptococci, or -hemolysin, from staphylococci) or by the host (e.g., the cytolysin of complement) cause cell injury and cell death. Our long term goal is the elucidation of the mechanisms of cell death caused by such agents.